This invention relates to a display device having a cathode ray tube which comprises a display screen and a deflection unit for deflecting electron beams, the cathode ray tube containing an in-line electron gun which includes a main lens portion having means for generating a main lens field and a quadripolar field, the display device having means for dynamically varying the intensity of the main lens field and the quadripolar field, the electron gun having means for generating, in front of the main lens field, a pre-focusing lens field and a further quadripolar field, and the display device having means for dynamically varying the intensity of the pre-focusing field and the further quadripolar field.
The invention also relates to a cathode ray tube which can suitably be used in a display device.
Display devices are used, inter alia, in TV receivers and colour monitors.
A display device of the type mentioned in the opening paragraph, and a cathode ray tube which can suitably be used in such a display device are known from European Patent Application EP-509590, which corresponds to U.S. Pat. No. 5,347,202.
In operation, the deflection unit generates an electromagnetic field for deflecting electron beams across a display screen. These electron beams are generated in the electron gun. The deflection field has a refocusing effect on the electron beams and causes astigmatism. These effects vary with the degree of deflection. The electron gun comprises means for generating a main lens field and a quadripolar field, and the display device includes means for dynamically varying the intensity of said main lens field and quadripolar field. By virtue thereof, astigmatism and focusing of the electron beams can be controlled as a function of the deflection in such a manner such that astigmatism caused by the deflection field is at least partially compensated for and focusing is at least substantially constant across the display screen. This has a positive effect on picture reproduction. In the literature, such electron guns are also referred to as DAF guns (Dynamic-Astigmatism and Focusing). To preclude disturbing Moire effects, particularly at the edges of the display screen, the display device known from EP-A-509590 comprises means for generating a dynamic pre-focusing field and a dynamic, further quadripolar field. In particular very small vertical spot dimensions at the edges of the display screen can be precluded. In the known display device, the dynamic pre-focusing field and the dynamic, further quadripolar field together constitute a dynamic cylindrical lens, which influences the beam diameter in the vertical direction, but has almost no influence in the horizontal direction. Within the scope of the invention, the term "quadripolar field" is to be understood to mean an electric field having a quadripolar component.